Vaccinia virus: There are several membranes that envelope vaccinia virus, each containing specific membrane proteins. The presence or absence of one or more membranes leads to alternate forms of the infectious virus. Within two of these membrane forms are membrane proteins to which a vigorous immune response is mounted. We are studying two of these proteins with the use of biochemical techniques and the methods of x-ray crystallography. Marburg virus: The matrix protein vp40 from the Marburg virus is thought to be a peripheral membrane protein. It makes up about one-third of the virus particle and has similarities to the vp40 protein of the related Ebola virus. We have determined the three-dimensional structure of the Marburg vp40 protein. The protein has two domains and is found in the crystal in an oligomeric state that may reflect how it is packed into the viral particle. The protein functions as a dimer of two protein molecules bound together and we are able to detail the interactions that allow this binding. By analysis of the Marburg vp40 structure with comparison to the known Ebola structure, we hope to gain insights into the function of vp40 in virus replication and pathogenicity.